In the year 1859, a book was published that forever changed people's understanding of themselves in relation to all other living things. The book was On the Origin of Species, and its author was Charles Darwin, a quiet English scientist who claimed that all living plants and animals had developed, or evolved, from earlier forms of life.

Before Darwin, most people believed that each of the more than a million species of plants and animals had been created separately and had remained unchanged since the earth's beginning. They believed that humans were not related to any other species of animal but had been specifically created to rule over all other living things. Darwin's research led him to discover very different ideas, however.

Charles Darwin was born on February 12, 1809, in Shrewsbury, England, into a wealthy and well-known family. His grandfather, Erasmus, was a famous scientist and poet. Charles attended an excellent school, but his record was far from brilliant. He did, however, show a strong interest in nature.

At 16, Charles was sent to Edinburgh University to study medicine. But watching surgery made him so ill that in 1828 he transferred to Cambridge University, where he studied for the clergy. His interest in plants and animals remained very strong.

Darwin graduated in 1831. Through his friendship with John Henslow, his botany professor, Darwin was appointed geologist on the naval vessel H.M.S. Beagle. He became the ship's naturalist. The Beagle set sail in 1831 on a five-year voyage around the world.

During the voyage of the Beagle, expeditions were made into South America, the Galápagos Islands, and other islands of the Pacific. Wherever he went, Darwin studied the geology. He searched for fossils. He observed plant and animal life. And he thought about relationships between living creatures and fossil animals. He kept a journal that was to become the basis for his theory of evolution.

On his voyage Darwin read Charles Lyell's book The Principles of Geology, which began to have a great influence on his thinking. Unlike most geologists before him, who believed that the face of the earth was the result of a few great and violent geological changes, Lyell saw that the earth was continually and slowly shaped by ordinary events. Simple processes, like waves and wind and rain, could act over vast periods of time to alter gradually the earth's features. These ideas were extremely important to the theory of evolution that Darwin soon was to develop.

In 1836, his earlier plans for the clergy forgotten, Darwin returned to London, where he wrote his account of the voyage. By now he was convinced that all forms of life had evolved from earlier forms. The question was: How did such evolution occur?

Domestic plants and animals, he noted, were bred for certain desirable features. English racehorses were an example of this. Starting long ago with ordinary horses, people carefully selected the strongest and swiftest animals to be bred together. After continuing this practice for many generations, they had a new type of horse — the sleek English racehorse. Selection by people was the key to breeding in domestic animals. Darwin was convinced that selection also took place in nature, but he wondered how.

Darwin realized that in all species of plants and animals there is a natural range of variation, with no two individuals exactly alike. Those individuals who are more likely to survive and produce offspring are those whose variations better suit them to their environment. Darwin called this sorting out of naturally favored variations natural selection. He realized that the result of natural selection over many generations would be a gradual change in the species.

Darwin did not publish his ideas on evolution immediately. In 1839 he married his cousin, Emma Wedgwood, and a few years later they moved to Down, Kent, in southern England. He was in poor health and settled down to a quiet life in the countryside.

Then in 1858, Darwin was sent an essay by Alfred Russel Wallace, an English naturalist. Darwin was stunned to find a theory of natural selection in this manuscript. Wallace's paper was ready for publication; Darwin's huge book — the result of years of work — was not.

Darwin quickly put together a research report that was presented with Wallace's paper at the Linnaean Society meeting in London in 1858. The following year Darwin published his great work, On the Origin of Species by Means of Natural Selection. This book contained both the theory of evolution and the evidence for it.

All 1,250 copies of the first edition were sold in a single day. Many people found Darwin's theory of evolution too unsettling and attacked it, and a great controversy followed. The book upset many established scientific ideas and also contradicted firmly held religious beliefs. But Darwin's ideas were supported by many respected scientists and after a few years Darwin's theory was accepted by much of the scientific community. In 1863, Darwin was awarded the Copley Medal of the Royal Society, Britain's highest scientific honor. During the following years he developed his ideas on evolution further in a number of books. In The Descent of Man, and Selection in Relation to Sex (1871) he discussed the evolution of humans, arguing that we along with the apes evolved from a common ancestral species.

Darwin died in Kent on April 19, 1882. He was honored for his achievements by being buried in England's Westminster Abbey, and to this day scientists continue the work he began.